A&P Something that has always confused me about lens

The f / number related to describing the speed of a lens. I've seen people rate this lens or that lens as a 1.8 or a 1.4 and tie that in with clarity and sharpness. I wouldn't think that a slower lens would necessarily have less clarity or sharpness. I would think that say a Canon ef mount L series lens at f4 would be clear and sharp compared to a tamron or sigma 50 or 85 mm at f1.8. What gives?

The lower the stop number, the more light the lens lets through, the faster you can shoot (i.e. 1/80 @ F4 vs. 1/250 @ f2.2)...

And as stated, generally, the lower the f stop, the more money you are going to spend on a lens, and the more likely it is going to be that the elements are going to be higher quality, thus producing sharper images through a broader f range....

Not really... I mean I can pick up a 50 mm f 1.8 for 80 bucks or so, I wouldn't think that would be able to produce an image any sharper than an f/4 L series for 1k.

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Then you would be mistaken. There are some magical lenses.

Nikon's least expensive lens, the 50/1.4, can take some of the sharpest pictures form their entire lens line. But it's only a 50 prime, and you have to know where the sweet spots are, even among different samples of the lens.

Nikon's least expensive lens, the 50/1.4, can take some of the sharpest pictures form their entire lens line. But it's only a 50 prime, and you have to know where the sweet spots are, even among different samples of the lens.

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Define sweet spot. Do you mean running through the aperture range taking photos with some sort of calibration card to determine at which aperture the lens produces the sharpest image?

The lower the stop number, the more light the lens lets through, the faster you can shoot (i.e. 1/80 @ F4 vs. 1/250 @ f2.2)...

And as stated, generally, the lower the f stop, the more money you are going to spend on a lens, and the more likely it is going to be that the elements are going to be higher quality, thus producing sharper images through a broader f range....

Clear as mud?

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I always thought that a lens being considered "fast" referred to its ability to focus quick, not how fast a shutter speed can be

are you talking about depth of field? if so, F1.4 is a faster aperture than F4 because it is a bigger hole (thus exposing the sensor to more light and allowing a faster shutter time - i.e. faster lens.) A byproduct of this "larger hole" is that the area of focus is more shallow than the area of focus provided by smaller holes (stopped down apertures). So, if you take an image at F1.4, you're going to have a shallower DOF (depth of field) than if you would have taken the same image at F4.

The clarity and sharpness you might be talking about is that natural effect of blurring out whatever areas of the image you're not focusing on. It's also called bokeh.

Also, the difference between say, the 50 F1.8 and the 50 F1.4 is quality, thus the huge price difference. "How sharp a lens is" greatly depends on the quality of the lens and its components. The F1.4 is a very sharp lens compared to the F1.8. HOWEVER, the 50mm F1.8 stopped down to F4 WILL provide the same depth of field as a 50mm F1.4 stopped down to F4. (it just might not be as sharp).

This is my understanding of this. If you have 2 lenses, say a 50mm 1.4 and a 50mm 2.8(example purpose only) The 1.4 has 2 stops more than the 2.8 and generally you get your sharpest images when you stop down 2 stops. So if you shoot the 1.4 @ 2.8 you are in that lenses sharpest range. So that same image would be sharper than a 50mm 2.8, the 2.8 lens would be sharpest around around 5.6

are you talking about depth of field? if so, F1.4 is a faster aperture than F4 because it is a bigger hole (thus exposing the sensor to more light and allowing a faster shutter time - i.e. faster lens.) A byproduct of this "larger hole" is that the area of focus is more shallow than the area of focus provided by smaller holes (stopped down apertures). So, if you take an image at F1.4, you're going to have a shallower DOF (depth of field) than if you would have taken the same image at F4.

The clarity and sharpness you might be talking about is that natural effect of blurring out whatever areas of the image you're not focusing on. It's also called bokeh.

Also, the difference between say, the 50 F1.8 and the 50 F1.4 is quality, thus the huge price difference. "How sharp a lens is" greatly depends on the quality of the lens and its components. The F1.4 is a very sharp lens compared to the F1.8. HOWEVER, the 50mm F1.8 stopped down to F4 WILL provide the same depth of field as a 50mm F1.4 stopped down to F4. (it just might not be as sharp).

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Not taking into account DOF at all. talking about overall clarity and sharpness of the entire image.

I always thought that a lens being considered "fast" referred to its ability to focus quick, not how fast a shutter speed can be

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it doesn't have anything to do with either of them, it's just an adjective that describes the f/stop. thus f/1.4 is "faster" than f/2.8. doesn't have anything to do with focusing speed or shutter speed, although you could compare the f/stop to shutter speed...f/1.4 is going to have a faster shutter speed than f/2.8.

Define sweet spot. Do you mean running through the aperture range taking photos with some sort of calibration card to determine at which aperture the lens produces the sharpest image?

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Something like that, yes. Except that you use your eye, not a program. Chances are someone out there has already foung the general range for the sweet spot, google. Otherwise set up a tripod, and take a frame of the same object starting 1 stop down from open, through like f/20 or something...

it doesn't have anything to do with either of them, it's just an adjective that describes the f/stop. thus f/1.4 is "faster" than f/2.8. doesn't have anything to do with focusing speed or shutter speed, although you could compare the f/stop to shutter speed...f/1.4 is going to have a faster shutter speed than f/2.8.

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You are wrong, sir. For correctly exposed frames, shutter speed is DIRECTLY proportionate to f/stop. You cannot shoot a correctly exposed frame without light, the lower the f number, the more light that is allowed in.

You are wrong, sir. For correctly exposed frames, shutter speed is DIRECTLY proportionate to f/stop. You cannot shoot a correctly exposed frame without light, the lower the f number, the more light that is allowed in.

I think he is referring to actual aperture size when describing this. He said that f 1.4 is going to have a faster shutter speed that an f2.8 which is partially true under certain circumstances. If using aperture priority mode then yes, the camera will select a higher shutter speed to compensate for the additional light entering the shutter as a result of a larger aperture opening.

This has nothing to do with the original intent of the thread , which was to find out if the speed of a lens (I.e. f 1.8 lens) is inversely proportional to perceived or noticeable clarity or sharpness (I.e. f4 = lower clarity than f1.4)

The lower the stop number, the more light the lens lets through, the faster you can shoot (i.e. 1/80 @ F4 vs. 1/250 @ f2.2)...

And as stated, generally, the lower the f stop, the more money you are going to spend on a lens, and the more likely it is going to be that the elements are going to be higher quality, thus producing sharper images through a broader f range....

This is my understanding of this. If you have 2 lenses, say a 50mm 1.4 and a 50mm 2.8(example purpose only) The 1.4 has 2 stops more than the 2.8 and generally you get your sharpest images when you stop down 2 stops. So if you shoot the 1.4 @ 2.8 you are in that lenses sharpest range. So that same image would be sharper than a 50mm 2.8, the 2.8 lens would be sharpest around around 5.6

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disagree. a 50mm f2.8 is often a macro design. they really prabably are sharper at 2.8 than the 1.4 is at 2.8. mine is that way. more like the 1.4 is at F8 to match the 2.8 wide open

often a slow lens, has an advantage for example the minolta series. to rank them in order from sharpest to soft in 50mm would go like this